Is it possible to adjust a AV-640 vertical antenna so that any length of coax cable will not change the SWR reading? This means that all the bands it's capable of will not have a change in reading. Is this asking too much for the antenna to do? Tnx Bill

The Hy-Gain AV-640 vertical is a clone of the Cushcraft R7000, R8 ... End-fed 1/2-wave vertical.you need to check that the matching box and counter poise wires are properly connected.The length of coax should not effect your SWR substantially.

Greg, substantially is a very troubling word for me regarding this antenna. It seems no matter how much I adjust each element on this antenna for a good SWR, the coax length changes everything quite a bit and I know it shouldn't. Bill

The length of coaxial cable should make NO difference on any antenna. If it does it means you have a common-mode current issue. That can be minimized by using a 1:1 current balun at the feed point.

If the feed line impedance doesn't match the load impedance than the length of coax will make a difference even if there is no common mode current.

Since this is a 50 ohm feed line feeding a supposed 50 ohm antenna, I'd suggest that the feed point impedance is no longer 50 ohms. It's unlikely that the coax is no longer 50 ohms but it's a remote possibility.

As others have mentioned, the matching box at the base of the antenna is probably compromised.

Copyright 2000-2017 eHam.net, LLC
eHam.net is a community web site for amateur (ham) radio operators around the world.
Contact the site with comments or questions.
WEBMASTER@EHAM.NETSite Privacy Statement